<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a {
            border-bottom: 1px solid black;
            height: 1px;
        }
        .outside {
            background-color: red;
            height: 300px;
            overflow: auto;
        }
        .inner {

            margin-top: 10px;
            background-color: blue;
        }
    </style>
</head>
<body>
<div class="a"></div>
<div class="outside">
    <div class="inner">
        aaaa
    </div>
</div>
</body>
</html>